1. Field of the Invention
This invention relates generally to formation fluid sampling, and more specifically to an improved reservoir fluid sampling module, the purpose of which is to bring high quality reservoir fluid samples to the surface for analysis.
2. The Related Art
The desirability of taking downhole formation fluid samples for chemical and physical analysis has long been recognized by oil companies, and such sampling has been performed by the assignee of the present invention, Schlumberger, for many years. Samples of formation fluid, also known as reservoir fluid, are typically collected as early as possible in the life of a reservoir for analysis at the surface and, more particularly, in specialized laboratories. The information that such analysis provides is vital in the planning and development of hydrocarbon reservoirs, as well as in the assessment of a reservoir""s capacity and performance.
The process of wellbore sampling involves the lowering of a sampling tool, such as the MDT(trademark) formation testing tool, owned and provided by Schlumberger, into the wellbore to collect a sample or multiple samples of formation fluid by engagement between a probe member of the sampling tool and the wall of the wellbore. The sampling tool creates a pressure differential across such engagement to induce formation fluid flow into one or more sample chambers within the sampling tool. This and similar processes are described in U.S. Pat. Nos. 4,860,581; 4,936,139 (both assigned to Schlumberger); U.S. Pat. Nos. 5,303,775; 5,377,755 (both assigned to Western Atlas); and U.S. Pat. No. 5,934,374 (assigned to Halliburton).
The desirability of housing at least one, and often a plurality, of such sample chambers, with associated valving and flow line connections, within xe2x80x9csample modulesxe2x80x9d is also known, and has been utilized to particular advantage in Schlumberger""s MDT tool. Schlumberger currently has several types of such sample modules and sample chambers, each of which provide certain advantages for certain conditions. None of these sample module/chamber combinations, however, exhibit all the characteristics of: permitting a gas charge behind the collected sample for better pressure management of the sample; being heatable up to 400xc2x0 F. at internal pressures up to 25,000 psi to promote the sample fluid components to go back into solution; being sized and certified for transportation directly from the well site to the laboratory without a need to transfer the collected sample; and being equipped to serve as a storage vessel. Nor do known sample chambers/modules sufficiently minimize the dead volume during sampling to reduce contamination of the sample by a pre-filling fluid, such as water.
To address these shortcomings, it is a principal object of the present invention to provide an apparatus and method for bringing a high quality formation fluid sample to the surface for analysis.
It is a further object of the present invention to provide a sample chamber that is safely heatable to at least 400xc2x0 F. at internal pressures up to 25,000 psi at the surface.
It is a further object of the present invention to provide a sample chamber that is able to be pressurized to maintain a sample in xe2x80x9csingle phase,xe2x80x9d meaning that as the sample cools down pressure must be maintained so that components such as gas and asphaltenes, which would normally separate out of the mixture during the pressure reduction caused by the cooling of the sample mixture, will remain in solution. Components that do not stay in solution by maintaining pressure while the sample cools, such as paraffins, can be recombined by applying heat to the chamber at the surface. It is a further object of the present invention to provide a sample chamber that is certified for transportation so that, if desired, the sample can be taken directly to a lab for analysis without the need for transferring the sample from the sample chamber at the wellsite.
It is a further object to provide a sample chamber that is adapted for use as a storage vessel, meaning the sample contents will not leak across the seals that contain the sample within the sample chamber.
It is a further object to provide a sample chamber having a volume that is adequate for proper PVT sampling, but not too large that the sample could not be transferred, if desired, into a separate transportable sample bottle, most of which are 600 cc or less in capacity.
It is a further object to provide an independent validation sample chamber, having a substantially smaller capacity than the sample chamber, that will be safer and easier to heat and recombine separated sample components on the surface for validating the quality of the sample at the well site.
The objects described above, as well as various other objects and advantages, are achieved by a sample module for use in a downhole tool to obtain fluid from a subsurface formation penetrated by a wellbore. The sample module includes a sample chamber carried by the module for collecting a sample of formation fluid obtained from the formation via the downhole tool, and a validation chamber carried by the module for collecting a substantially smaller sample of formation fluid compared to the sample chamber. The validation chamber is removable from the sample module at the surface without disturbing the sample chamber.
The sample chamber and the validation chamber may be placed in either parallel or serial fluid communication with a fluid flowline in the downhole tool such that the chambers may be filled either substantially simultaneously or consecutively as desired.
Preferably, the sample chamber is adapted for maintaining the sample stored therein in a single phase condition as the sample module is withdrawn with the downhole tool from the wellbore. The phrase xe2x80x9csingle phasexe2x80x9d is used herein to mean that the pressure of the sample within a chamber is maintained or controlled to such an extent that sample constituents which are maintained in a solution through pressure only, such as gasses and asphaltenes, should not separate out of solution as the sample cools upon withdrawal from the wellbore. The sample may be reheated at the surface to recombine the constituents which have come out of solution due to cooling, such as paraffins. Alternatively, the validation chamber may also be adapted for maintaining the fluid sample stored therein in a single phase condition as the sample module is withdrawn from the wellbore.
It is also preferred that the sample chambers be capable of safely withstanding heating at the surface, following collection of samples and withdrawal of the sample module from the wellbore, to temperatures necessary to promote recombination of the sample components within the chambers that may have separated due to cooling upon withdrawal.
It is further preferred that the sample chamber be sufficiently equipped so as to be certified for transportation.
Still further, it is desirable that the sample chamber be adapted for storing the sample collected therein for an indefinite period without substantial degradation of the sample. One solution for achieving this goal is for the sample chamber to include metal-to-metal seals as the final shut-off seals for the sample collected therein.
In another aspect, the present invention provides an improved sample chamber for use in a downhole tool to obtain fluid from a subsurface formation penetrated by a wellbore. The improved sample chamber includes a substantially cylindrical body capable of safely withstanding heating at the surface, following collection of a formation fluid sample via the downhole tool and withdrawal of the sample chamber from the wellbore, to temperatures necessary to promote recombination of the sample components within the chambers. Additionally, the body is sufficiently equipped so as to be certified for transportation. At least one floating piston is slidably positioned within the body so as to define a fluid collection cavity and a pressurization cavity, whereby the pressurization cavity may be charged to control the pressure of the sample collected in the collection cavity. A second such piston may be provided to create a third cavity wherein a buffer fluid may be utilized during sample collection. Metal-to-metal seals act as the final shut-off seals for the sample collected in the collection cavity of the body.
In another aspect, the present invention provides an apparatus for obtaining fluid from a subsurface formation penetrated by a wellbore. The apparatus includes a probe assembly for establishing fluid communication between the apparatus and the formation when the apparatus is positioned in the wellbore, and a pump assembly for drawing fluid from the formation into the apparatus. A sample chamber is provided for collecting a sample of the formation fluid drawn from the formation by the pumping assembly, and a validation chamber is provided for collecting a substantially smaller sample of the formation fluid than the sample chamber. The validation chamber is removable from the apparatus at the surface without disturbing the sample chamber or its contents.
It is preferred that the sample chamber be adapted for maintaining the sample stored therein in a single phase condition as the apparatus is withdrawn from the wellbore. In this regard, the sample chamber may include at least one floating piston slidably positioned within the sample chamber so as to define a fluid collection cavity and a pressurization cavity. A flow line in the apparatus establishes fluid communication between the probe assembly, the pump assembly, and the fluid collection cavity of the sample chamber. A pressurization system in the apparatus charges the pressurization cavity to control the pressure of the collected sample fluid within the collection cavity via the floating piston. The pressurization system preferably includes a valve positioned for fluid communication with the pressurization cavity of the sample chamber, the valve being movable between positions closing the pressurization cavity and opening the pressurization cavity to a source of fluid at a greater pressure than the pressure of the formation fluid delivered to the collection cavity.
The pressurization system controls the pressure of the collected sample fluid within the collection cavity during either collection of the sample from the formation, or retrieval of the apparatus from the wellbore to the surface, or both. For the former purpose, the source of fluid at a greater pressure than the pressure of the collected sample fluid may be wellbore fluid. For the latter purpose, the source of fluid at a greater pressure than the pressure of the collected sample fluid may be a source of inert gas, such as Nitrogen, carried by the apparatus.
The apparatus may be a wireline-conveyed formation testing tool, but is not necessarily so limited.
In another aspect, the present invention contemplates a method for obtaining fluid from a subsurface formation penetrated by a wellbore, and includes the steps of positioning an apparatus within the wellbore, establishing fluid communication between the apparatus and the formation, and inducing movement of fluid from the formation into the apparatus. A sample of the formation fluid moved into the apparatus is delivered to a sample chamber for collection therein, and a substantially smaller sample of the formation fluid moved into the apparatus is delivered to a validation chamber for collection therein. This permits the smaller sample to be evaluated independently of the sample stored in the sample chamber following withdrawal of the apparatus from the wellbore to recover the collected samples.